1. Field of Invention
The present invention is directed to step mask having a plurality of test cells and a method of producing the same. Each test cell of the step mask is etched for a different amount of time and therefore has a different etch depth or height. The number of phase shifter layer etch iterations can be conducted on a column-by-column and row-by-row basis to decrease the number of etch iterations.
2. Description of Related Art
Photomasks are used in the semiconductor industry to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide wafer. In its simplest form a photomask is comprised of a transparent substrate and a patterned layer opaque material, the pattern of the opaque material being a scaled negative of the image desired to be formed on the semiconductor wafer.
To create an image on a semiconductor wafer, photomask is interposed between an undeveloped semiconductor wafer which includes a layer of photosensitive material and an energy source commonly referred to as a Stepper. Energy from the Stepper is inhibited from passing through the areas of the photomask in which the opaque material is present. However, energy generated by the Stepper passes through the portions of the substrate of the photomask not covered by the opaque material and causes a reaction in the photosensitive material on the semiconductor wafer. Through subsequent processing the image created on the photosensitive material is transferred to the silicon or gallium arsenide wafer.
As circuit densities on semiconductor wafers have continued to increase and the minimum feature size on semiconductor wafers have continued to shrink, optical lithography has entered the sub-wavelength regime and is approaching its resolution limits. Extending the use of optical Steppers into deep sub-wavelength has necessitated the development of new technologies such as phase shift lithography and the use of phase shift masks.
In general, phase shift lithography uses the interference of light rays to overcome diffraction and improve the resolution of optical images projected onto a semiconductor wafer. There are currently two general types of PSMs. The first type of PSM is an embedded attenuating phase shift mask (EAPSM) which is comprised of an etched phase shifter layer (e.g., MoSi) and a quartz substrate. The second type of PSM is an alternating aperture phase shift mask (AAPSM) which is comprised of a lawyer of opaque material (e.g., chromium) and an etched quartz substrate. For EAPSMs, it is critical to create correct phase and transmission characteristics, and in order to achieve correct phase and transmission characteristics a very definitive end point has to be achieved during the MoSi etching processes. Further, a clear understanding of how phase, transmission, and etch rate depend on etch depth is important. However, optical properties and chemical stoichiometry of the phase shifter layer of PSMs are non-homogenous. Consequently, index of refraction, extinction coefficient and etch rate vary across the phase shifter layer of PSMs. Furthermore, PSM blanks are different from supplier to supplier, each having a different thickness of phase shifter layer.
Traditional methods used to characterize the phase, transmission, and etch rate of PSMs and their correlation to etch depth require multiple etching steps with the transmission and etch depth being measured after each etch step. The major disadvantages of these methods include (1) long lead time because of measurements made after each etch step, (2) loss of information from the previous etch step after each new etch step is performed, (3) phase measurements cannot be obtained prior to the Cr being striped, and (4) high costs since multiple masks are normally required. Further, the effects of phase and transmission on metrology and inspection tools is are still unclear. Currently there is no known standard for critical dimension (CD) measurement of PSMs, since the full effects of phase and transmission must be understood before a standard can be established.
Finally, it is not fully understand how phase and transmission effects the images created on semiconductor wafers requiring the study of PSMs having various phase and transmission scenarios. Although multiple phase shift masks can be used for such study, this would be costly and would introduce the additional variable of PSM blank variation. Therefore, if various phase and transmissions can be produced on a single PSM, it would greatly increase efficiency and lower costs.
Accordingly, it is the object of the present invention to provide a step mask having a plurality of test cells or sites of various etch depths created using a process set forth below which overcomes the disadvantages described above with respect to methods known in the prior art.
It is a further object of the present invention to provide a test mask which can be used to study the phase effect on critical dimension measurements and inspection and wafer printability.
It is a further object of the present invention to provide a step mask that can be produced with a short turnaround time.
It is yet another object of the present invention to provide a step mask which preserves information for each etch step.
It is yet another object of the present invention to provide a step mask which enables measurement efficiency since information on various etch depths or heights can be taken at the same time on a single mask.
It is still a further object of the present invention to provide a step mask that can be used as a standard for phase and transmission correlation with etch depth.